System, process and related sintered article

ABSTRACT

A system, process and related sintered article are provided. The process includes supporting a piece of inorganic material with a pressurized gas and sintering the piece of inorganic material while supported by the pressurized gas by heating the piece of inorganic material to a temperature at or above a sintering temperature of the inorganic material such that the inorganic material is at least partially sintered forming the sintered article. The inorganic material is not in contact with a solid support during sintering. The sintered article, such as a ceramic article, is thin, has high surface quality, and/or has large surface areas.

This application claims the benefit of priority under 35 U.S.C. § 119 ofU.S. Provisional Application Ser. No. 62/287,070, filed on Jan. 26,2016, the contents of which are relied upon and incorporated herein byreference in its entirety.

BACKGROUND

The disclosure relates generally to formation of a sintered inorganic orceramic material, and specifically to systems and processes forsintering inorganic material in a non-contact environment as well as thesintered articles, such as ceramic sheets or tapes, made from suchsystems and processes.

Articles, such as thin sheets, tapes, or ribbons of ceramic have manypotential uses, such as serving as waveguides, when the ceramic istransmissive to light, serving as substrates that may be coated orlaminated, and integrated in batteries and other components, or otherapplications. Such articles are typically manufactured by forming largeingots of the sintered material, cutting wafers, slabs or plates of thematerial, and polishing the corresponding articles to a desired form andsurface quality. Polishing helps to remove flaws or defects on thesurfaces of the articles, but is time and resource intensive. Sucharticles may also be manufactured by tape casting, gel casting, or otherprocesses that include sintering of green tapes, such as strips ofinorganic grains bound in an organic binder. In such conventionalprocesses, the green tapes are typically placed upon a surface, called asetter board, and placed inside a furnace that burns off the organicbinder and sinters the inorganic grains. The setter board is typicallyformed from a refractory material that can withstand the sinteringprocess, namely it does not react with or bond to the article that isbeing fired. The setter board supports the tape when the binder isremoved, and at least one surface of the remaining inorganic material isin contact with the setter board during sintering.

SUMMARY

One embodiment of the disclosure relates to a process of forming asintered article. The process includes supporting a piece of inorganicmaterial with a pressurized gas. The process includes sintering thepiece of inorganic material while supported by the pressurized gas byheating the piece of inorganic material to a temperature at or above asintering temperature of the inorganic material such that the inorganicmaterial is at least partially sintered forming the sintered article,and at least a portion of the inorganic material being sintered is notin contact with a solid support during sintering.

One embodiment of the disclosure relates to a sintered article. Thesintered article includes a first major surface, a second major surfaceopposite the first major surface, an at least partially sinteredinorganic material defining the first major surface, the second majorsurface and a body extending between the first and second majorsurfaces. The sintered article includes an average thickness between thefirst and second major surfaces that is no more than 1 mm. The sinteredarticle includes a width defined as a first dimension of one of thefirst or second major surfaces orthogonal to the thickness. The sinteredarticle includes a length of the sintered article defined as a seconddimension of one of the first or second major surfaces orthogonal toboth the thickness and the width of the sintered article. The sinteredarticle is thin such that at least one of the widths and the lengths isgreater than five times the average thickness. The first major surfaceis defined by a first surface quality metric and the second majorsurface is defined by a second surface quality metric. The first surfacequality metric is substantially the same as the second surface qualitymetric. The inorganic material is selected from the group consisting ofpolycrystalline ceramic and synthetic mineral.

An additional embodiment of the disclosure relates to a system forforming a sintered article. The system includes a tape supply of a tapematerial comprising grains of an inorganic material bound by an organicbinder. The system includes a binder removal station following the tapesupply. The system includes a sintering station following the binderremoval station. The system includes a heating system heating the binderremoval station and the sintering station. The heating system heats thebinder removal station to a temperature between 100 and 400 degreesCelsius such that the organic binder is pyrolyzed as the tape materialtraverses the binder removal station. The heating system heats thesintering station to a temperature greater than 800 degrees Celsius suchthat the inorganic material at least partially sinters as the tapematerial traverses the sintering station. The system includes a gasbearing supporting the tape material within the binder removal stationand the sintering station.

Additional features and advantages will be set forth in the detaileddescription that follows, and, in part, will be readily apparent tothose skilled in the art from the description or recognized bypracticing the embodiments as described in the written description andclaims hereof, as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description are merely exemplary, and areintended to provide an overview or framework to understand the natureand character of the claims.

The accompanying drawings are included to provide a furtherunderstanding and are incorporated in and constitute a part of thisspecification. The drawings illustrate one or more embodiment(s), andtogether with the description serve to explain principles and theoperation of the various embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sintering system according to an exemplary embodiment.

FIG. 2 is a plot of temperature vs. position within the sintering systemof FIG. 1 according an exemplary embodiment.

FIG. 3 is a sintering station according to an exemplary embodiment.

FIG. 4 is a cross-sectional view of the sintering station of FIG. 3according to an exemplary embodiment.

FIG. 5 is the sintering station of FIG. 3 with the gas upper bearingstructure removed according to an exemplary embodiment.

FIG. 6 is a plot of temperature vs. position within the sinteringstation of FIG. 3 according to an exemplary embodiment.

FIG. 7 is a sintered article according to an exemplary embodiment.

FIG. 8 is a cross-sectional view of the sintered article of FIG. 7according to an exemplary embodiment.

FIG. 9 is a venting gas bearing according to an exemplary embodiment.

FIG. 10 is a gas bearing plenum according to an exemplary embodiment.

FIGS. 11 and 12 show an air bearing supporting a tape material duringbinder burn-off according to an exemplary embodiment.

FIG. 13 shows a gas bearing arrangement according to another exemplaryembodiment.

FIG. 14 shows an air bearing surface of an air bearing, according to anexemplary embodiment.

FIG. 15 shows a side view of an air bearing, according to an exemplaryembodiment.

FIGS. 16 and 17 show photographs of sintered zirconia tape sinteredusing different air bearing gap sizes, according to an exemplaryembodiment.

FIGS. 18 and 19 show photographs of sintered alumina tape sintered usingdifferent air bearing gap sizes, according to an exemplary embodiment.

FIGS. 20 and 21 show photographs of Pyrex tape material before and aftersintering, according to an exemplary embodiment.

FIGS. 22 and 23 show photographs of sintered zirconia tape materialshowing the effect of application tension to the material duringsintering, according to an exemplary embodiment.

DETAILED DESCRIPTION

Referring generally to the figures, various embodiments of a sinteredarticle (such as a ceramic article, tape or sheet) and a system andmethod for forming a sintered article are shown. In various embodimentsdiscussed herein, the sintered article is formed, at least in part, in anon-contact sintering process. In various embodiments, this non-contactsintering is provided by a system that supports a piece, strip or tapeof inorganic material with a gas bearing while heat is being deliveredto the inorganic material to cause sintering. In this arrangement,neither of the major surfaces of the piece of inorganic material aresupported by or in contact with a solid support (such as a setter board)during sintering.

Applicant believes that this non-contact sintering process enablesformation of higher quality sintered articles, such as ceramic articles,than contact-based processes. Because both major surface of theinorganic material experience essentially the same conditions due to thegas bearing support, sintered articles as discussed herein may be formedhaving very high surface qualities and highly consistent surfacesqualities between the first and second major surfaces. For example,because both surfaces are only contacted by the gas (e.g., air) from thegas bearing neither of the major surfaces are altered by chemical orparticulate contaminants as is typically the case with the surface of aceramic article sintered on a setter board. As another example, becausethe inorganic material shrinks or contracts during sintering, thenon-contact sintering process discussed herein allows for frictionless,unconstrained shrinkage or growth during all stages of the sinteringprocess thereby preventing development of stresses and eliminatingformation of scratches or abrasions present in articles sintered on asetter-board. This lack of solid contact during sintering provides asintered article with high levels of surface flatness, low surfacedefect levels, and low surface roughness levels in the as-formedarticle, without the need to polish the major surfaces of the article.

Further, Applicant believes that due to the reduction or elimination ofthe stress, friction and abrasion common in setter board based sinteringsystems, the non-contact sintering system discussed herein enables theformation of large area sintered articles, not believed obtainable withthe prior sintering techniques. In particular, the systems discussedherein enable the formation of large area sintered articles having thehigh quality surfaces discussed above without the need for polishing. Inaddition, Applicant believes that the systems discussed herein enablethe formation of sintered articles that are very thin and with majorsurfaces that are large in surface area.

Further, in specific embodiments discussed herein, the inorganicmaterial is sintered while supported on both major surfaces by gasbearings. Applicant believes that by applying non-solid contact pressureto both major surfaces of the piece of inorganic material via the gasbearing during sintering, even higher quality and/or larger sinteredarticles can be produced. For example, by pressing on both majorsurfaces with the gas bearing during sintering, Applicant believes thatproblems such as cracking, curling and warping common during sinteringcan be reduced or eliminated even in large area sintered articleswithout introducing the problems (e.g., surface defects, contaminants,etc.) presented by solid surface support as discussed above. Further,the pressure provided on both major surfaces by the gas bearing may alsofunction to support, compact or generally hold together the inorganicmaterial following pyrolysis of a binder material while the inorganicmaterial is being moved into a sintering station and during sintering.

Referring to FIG. 1, a system 10 and related method for sintering apiece of inorganic material is shown. In general, system 10 includes asupply 12 of inorganic material, shown as tape 14. In general, tape 14is an elongate piece of material including one or more inorganicmaterial bound together with an organic binder material, and in general,as tape 14 traverses the various sections of system 10, most or all ofthe binder material of tape 14 is removed (e.g., via burning and/orcharring) and the inorganic material is at least partially sinteredforming a sintered article or material, shown as sintered material 16,at the output of system 10. In various embodiments supply 12 is anysource of tape 14. In various embodiments, supply 12 may be a reel orspool of elongate tape 14 that is unwound as tape 14 is moved throughsystem 10. In other embodiments, supply 12 may be a system that formstape 14 within system 10 (e.g., inline with the downstream componentsdiscussed below). In various embodiments, tape 14 includes particles ofa wide variety of materials, including ceramics, glass and/or metals,held together by an organic binder, and in specific embodiments, tape 14is made processes such as tape casting, calendaring, extrusion, andcombinations thereof.

In general, system 10 includes a binder removal station or zone 18, aheating station or zone 20, a sintering station or zone 22 and a coolingstation or zone 24. System 10 includes a gas bearing system thatincludes upper and lower gas bearings, shown as gas bearings 26 and 28,and a channel 30 defined by opposing bearing surfaces 32 and 34. Gasbearings 26 and/or 28 include posts or stand-offs 35 that supports andspaces gas bearings 26 and 28 from each other. In general, upper andlower gas bearings 26 and 28 include a plurality of pores or nozzles 36(shown in FIGS. 4 and 5) that deliver gas into a channel 30 definedbetween opposing bearing surfaces 32 and 34. Gas supplied to channel 30via gas bearings 26 and 28 is supplied at a large enough rate orpressure such that a inorganic tape 14 and sintered material 16 issupported within bearing channel 30 such that they do not come intocontact with bearing surfaces 32 and 34 while being processed in system10. In this arrangement, gas bearings 26 and 28 extend between binderremoval station 18, through heating station 20, sintering station 22 andcooling station 24 such that neither of the major surfaces of the tapesare contacted during traversal of these stations.

In at least some embodiments, system 10 may include insulatingseparators located between zones 18, 20, 22 and 24 to isolate the zonesfrom each other and limit heat transfer between zones. In variousembodiments, system 10 may include unheated or room-temperatureseparators zones located between zones 18, 20, 22 and 24, andspecifically located between the gas bearings 26 and 28 of each zones.In some such embodiments, the separator zones may include a tensioningdevice applying tension to tape 14 or ceramic sheet 16 such that tape 14or ceramic sheet 16 is supported and guided across the separator zone,and in some such embodiments, the separator zones are non-gas bearingsupport sections. However, in other embodiments, the room temperatureseparator zones include gas bearings that support tape 14 or ceramicsheet 16 as it traverses the separated section.

As shown in FIG. 1, system 10 includes a plurality of different gasbearing sections that are aligned to form a straight channel 30extending through system 10. In particular embodiments, alignment of thegas bearing elements is important to ensure that channel 30 is straightsuch that the forces provided by gas bearings 26 and 28 are consistentalong the length of system 10 to eliminate unwanted shifting (e.g.,lateral shifting) that may otherwise occur at the transitions betweenthe various zones of system 10. In one embodiment, system 10 includes asupporting system that monitors positions of the gas bearing sectionsvertically and horizontally relative to each other and that allows foralignment of the gas bearing sections either manually or automaticallyto provide kinematic control and stability.

In yet other embodiments, system 10 may include additional zones orstations. In one such embodiment, system 10 includes a zone for apost-sintering heat treatment such as annealing.

Gas bearings 26 and 28 may be any suitable gas or air support system. Invarious embodiments, the gas bearing bodies may be solid or porous inconfiguration. Suitable materials for the gas bearing bodies include,but are not limited to, aluminum, bronze, carbon or graphite, stainlesssteel, high temperature alloys like UNS N06333 and UNS N06025, platinumand its alloys, ceramics like silicon carbide, etc. The gas bearing bodydimensions may be designed based on the shape or size of tape 14 andsintered material 16 and to efficiently and effectively support tape 14and sintered material 16 within bearing channel 30. In the case wherethe bearing bodies are porous, they may still include additionalapertures or holes for flowing gas and/or may use the porous structureto provide flow.

In various embodiments, gas flow rates, gas compositions and/ortemperatures can be independently controlled for zones 18, 20, 22, and24. In some embodiments, gas bearings 26 and 28 are configured todeliver an oxidative gas (e.g., atmospheric air or oxygen) to both sidesof tape 14 and sintered material 16. However, gas bearings 26 and 28 maybe configured to deliver any gas desired for different applications. Insome embodiments, nitrogen, helium, neon and/or argon may be deliveredvia gas bearings 26 and 28. In some embodiments, a reactive gas or a gascarrying an additive, dopant, reactant, etc., may be delivered via gasbearings 26 and 28 to add a material or to react with the material oftape 14 during processing, such as during sintering. In someembodiments, gas bearings 26 and 28 are configured to deliver a reducinggas, an inert gas or a forming gas to channel 30. In various embodimentsthe gas supplied by gas bearings 26 and 28 has been filtered, humidifiedor dried. In some embodiments, a first gas may be delivered via gasbearing 26 and a second, different gas may be delivered via gas bearing28, which in such embodiments, may allow for the formation of sinteredmaterial 16 having different properties on the opposing first and secondmajor surfaces.

System 10 includes a heating system, which in the embodiment shown,includes a plurality of heating elements 38 located within gas bearings26 and 28. Heating elements 38 deliver heat to each of the stations ofsystem 10 to process tape 14 into sintered material 16 as discussed inmore detail below. Heating elements 38 are in communication with thesolid portions of the bearing bodies of bearings 26 and 28 within zones18, 20 and 22 and/or with the air delivered to the heated portion ofbearing channel 30. In specific embodiments, heating elements 38 may beresistive heating elements located within or embedded in the heatedportions of bearings 26 and 28 such that the gas being delivered tobearing channel 30 within stations or zones 18, 20 and 22 is heated tothe desired temperatures. Heating elements 38 may be part of any type ofsuitable heating system to control the temperatures as discussed herein.In various embodiments, heating elements 38 may be part of an oven orfurnace delivering heat, such as hot air, to gas bearings 26 and 28. Insome embodiments, where very high sintering temperatures (e.g., 1800degrees Celsius or greater) are needed, heating elements 38, at leastwithin sintering zone 22, may be one or more plasma torch. Heat may besupplied by other methods such as by induction or with use of acombustible gas mixture. The rate of flow of the pressurized gas mayalso be adjusted for controlled cooling.

In one exemplary embodiment, heating elements 38 may be resistiveheating wires formed from a platinum-rhodium material, and in a specificembodiment, heating elements 38 are 80% platinum, 20% rhodium wires. Insome such embodiments, the heating elements 38 reside within aluminatubing located within air bearings 26 and 28. In a specific embodiment,the platinum-rhodium heating elements have a diameter of 0.040 inches.Applicant has built and tested an air bearing utilizing such heatelements to heat with 2.5 kW to over 1000 degrees C. In specificembodiments, heating elements 38 are powered with a power source of 100Amps at 100 VDC.

Referring to FIG. 1 and FIG. 2, processing of tape 14 into sinteredmaterial 16 is shown. Tape 14 is fed from tape supply 12 into binderremoval station 18, and the heating elements 38 of binder removalstation 18 heat tape 14 to a sufficient temperature such that most orall of the organic binder of tape 14 is removed via a process, such aspyrolysis, as tape 14 traverses binder removal station 18 leaving aself-supporting tape or sheet of the inorganic material of tape 14. Invarious embodiments, heating elements 38 heat binder removal station 18to a temperature between 100 and 400 degrees Celsius sufficient topyrolyze the organic binder material. In various embodiments, a largepercentage, e.g., more than 50%, more than 70%, more than 90%, more than99%, of the organic binder of tape 14 is removed as tape traversesbinder removal station 18.

In various embodiments, tape 14 entering binder removal station 18 is agreen tape material including inorganic material bound together with anorganic binder. According to exemplary embodiments, tape 14 includesinorganic material, such as polycrystalline ceramic and/or minerals(e.g., alumina, zirconia, lithium garnet, spinel), bound by an organicbinder (e.g., polyvinyl butyral, dibutyl phthalate, polyalkyl carbonate,acrylic polymers, polyesters, silicones, etc.). In contemplatedembodiments, tape 14 includes inorganic material, such as metalparticles, bound in an organic binder. In other contemplatedembodiments, tape 14 includes inorganic material, such as glass grains(e.g., high purity silica grains, borosilicate, aluminosilicate, sodalime) or other inorganic grains bound by an organic binder. Incontemplated embodiments, tape 14 includes inorganic material, such asglass-ceramic particles (e.g. cordierite, LAS lithium aluminosilicates,Nasicon structure lithium metal phosphates, celsian), bound in anorganic binder. According to an exemplary embodiment, tape 14 has aporosity of from about 0.01 to about 25 vol % and/or the inorganicparticles have a median particle size diameter of from 50 to 1,000nanometers and a Brunauer, Emmett and Teller (BET) surface area of from2 to 30 m²/g. In other contemplated embodiments, the above materials maybe bound by inorganic binders or other binders and/or the abovematerials may be otherwise sized or have other porosity.

Following binder removal, tape 14 moves into heating station 20. Heatingelements 38 within heating station 20 generates heat such that thetemperature within heating zone 20 gradually rises along the length ofheating station 20. In this manner, heating station 20 acts as apreliminary heating zone which gradually raises the temperature of tape14 to a temperature close to the temperature needed for sintering, andin some embodiments, begins the sintering process. In some embodiments,the gradual temperature increases across heating station 20 limits theformation of defects, cracks, etc. that may otherwise be formed bymoving tape 14 directly into the high temperature sintering station. Invarious embodiments, heating zone 20 raises the temperature of tape 14to a temperature of at least 70%, specifically at least 90%, and morespecifically at least 95%, of the temperature within sintering zone 22.In addition, heat provided by heating zone 20 removes some or allresidual binder material remaining in tape 14 entering zone 20.

As will be understood, following binder removal station 18, theinorganic material of tape 14 is no longer bound by the organic bindermaterial, but is substantially unsintered prior to processing throughsintering station 22. Because tape 14 following binder removal no longerincludes significant levels of binder material, one of ordinary skill inthe art may expect tape 14 following binder removal to simply collapseor fall apart under its own weight. However, Applicant has discoveredthat the tape 14 following binder removal remains intact, despite thebinder being burned off and/or charred, if the tape 14 is properlyhandled. In some embodiments, Applicant believes that pressure appliedby gas bearings 26 and 28 to both sides of tape 14 facilitate handlingand processing through system 10 following binder removal by holdingtogether the inorganic material of tape 14 after removal of the bindermaterial. In a specific embodiment, gas pressure provided by gasbearings 26 and 28 is applied in either uniaxial or isostatic manner toaid in consolidation and pore removal and speed sintering. In variousembodiments, gas bearings 26 and 28 are configured to provide gas atpressures between 0.5 and 200 psig.

Following heating zone 20, tape 14 moves into sintering zone 22. Ingeneral, within sintering zone 22, heating elements 38 generate heatsuch that tape 14 is heated to a temperature sufficient to sinter theinorganic material of tape 14 to form a sintered article, such assintered material 16. As will be understood, heating of the inorganicmaterial of tape 14 densities or reduces the porosity of the inorganicmaterial. The degree or amount of sintering achieved in sinteredmaterial 16 is a function of the temperature and time that tape 14spends within sintering zone 22. Further, the sintering temperatureprovided by sintering zone 22 also depends on the inorganic materialbeing sintered, and in general, heating elements 38 raise sintering zone22 to a temperature greater than 800 degrees Celsius and maintains thistemperature substantially constant over the length of sintering zone 22.In particular embodiments, the temperature of sintering zone 22 variesless than 30%, specifically less than 10% and more specifically lessthan 5% over the length of sintering zone 22.

In specific embodiments, sintering zone 22 is configured to at leastpartially sinter the inorganic material of tape 14, such as thepolycrystalline ceramic or other inorganic material of tape 14. Forexample, in specific embodiments, polycrystalline ceramic grains may besintered with in sintering zone 22 such that the grains bond or fuse toone another forming a sintered material 16 that includes a large amountof porosity (e.g., at least 10% by volume, at least 30% by volume),where the “porosity” refers to the portions of the volume of the tapeunoccupied by the inorganic material, such as the polycrystallineceramic.

Following sintering within zone 22, sintered material 16 enters coolingzone 24 which cools sintered material 16 to a temperature low enoughthat sintered material 16 can be moved, contacted, stored, etc. withoutdamaging the material. In one embodiment, cooling zone 24 cools tape toa temperature below 150 degrees Celsius. In the particular embodimentshown, sintered material 16 exits cooling station 24 and is stored in astorage location, such as uptake reel 40. Thus, in such embodiments, byutilizing the non-contact gas bearings, system 10 enables the formationof long pieces of high quality sintered material 16 in a substantiallycontinuous fashion. As will be noted from FIG. 1, system 10 enables theformation of sintered material 16 having a length longer than any one ofthe sections of system 10 and longer than stations 18, 20, 22 and 24 ofsystem 10. This is contrast to conventional, batch process ceramicsintered on a solid support, such as a setter board, which is sizelimited by both the setter board and the heating furnace typical in suchprocesses.

FIG. 1 shows system 10 arranged as a generally horizontal, linearprocessing system. In other embodiments, system 10 is verticallyarranged or arranged at a sloped angle relative to the horizontal plane.

Referring to FIGS. 3-5, sintering zone 22 and sintering of tape 14 isshown and described in more detail. As tape 14 traverses sintering zone22, the inorganic material of tape 14 is sintered such that sinteredmaterial 16 exits the output side of sintering zone 22. Within sinteringzone 22, tape 14 enters a first or input end 42 of channel 30, and theheat within sintering zone 22 causes the inorganic material of tape 14to sinter (e.g., densify, decrease porosity, etc.) as the tape 14 movestoward output end 44 of channel 30. In this arrangement, the density ofthe inorganic material present in sintered material 16 exiting outputend of channel 30 is greater than the density of the inorganic materialof tape 14 entering input end 42 at the start of sintering zone 22.Further, as shown in FIG. 3, the continuous nature of system 10 and therelated process allows for the sintering of material that has arelatively long length, such that the length of both tape 14 andsintered material 16 in the processing direction, shown as arrow 46, isgreater than the length of sintering zone 22 in the processing directionbetween input end 42 and output end 44 of channel 30.

As shown best in FIG. 5, sintering within sintering zone 22 also causescontraction or shrinkage of tape 14, such that a width, W1, of sinteredmaterial 16 exiting sintering station 22 is less than a width, W2, oftape 14 entering sintering station 22. In various embodiments, theshrinkage during sintering is substantial, such that W1 is less than 90%of W2, specifically less than 80% of W2, and more specifically less than75% of W2. As will be understood contraction during sintering alsooccurs in the thickness direction and in the length direction, and invarious embodiments, the length contraction is accounted for by a slowerlinear rate of output of the tape than the rate at which it is fed intothe system. Because this contraction occurs while tape 14 is supportedby gas bearings 26 and 28, the major surfaces of sintered material 16are high quality surfaces with low levels of defects. In contrast totypical setter board based sintering processes which introduce differentdefects such as abrasions, drag marks, pits, tears, etc. caused as thecontracting inorganic material is pulled across the solid setter boardsurface during sintering, sintered material 16 includes very few or nodefects resulting from the contraction during sintering.

Referring to FIG. 4, the arrangement of gas bearings 26 and 28 withinsintering zone 22 are shown in more detail. In particular, gas bearings26 and 28 are spaced from each other such that the gap size betweenopposing bearing surfaces 32 and 34 is sufficient to support tape 14during sintering. In this arrangement, gas bearings 26 and 28 providegas to channel 30 on both sides of tape 14. In this arrangement, notonly do gas bearings 26 and 28 support tape 14 in a noncontactenvironment during sintering, but gas bearings 26 and 28 also apply gaspressure to both of the major surfaces of tape 14 (e.g., upper and lowersurfaces in the orientation of FIG. 4) such that deformation (e.g.,curling, cracking, etc.) of tape 14 is limited or eliminated duringbinder removal and/or during sintering.

Further, gas bearings 26 and 28 may also be configured to apply pressureto both of the major surfaces of tape 14 to shape, compact or flattentape 14 during sintering. In some embodiments, bearing surfaces 32 and34 may be shaped, curved, corrugated, etc. imparting the correspondingshape to sintered material 16. In addition, curved bearing surfaces 32and 34 may be utilized to limit or prevent warping or bow in sinteredmaterial 16 in the direction across sintered material 16, perpendicularto the direction of movement through system 10. Sintered material 16with arced, corrugated, or other one-dimensionally shaped profiles canbe made by the continuous process starting with a flat tape or sheet asthe input using a high temperature gas bearing with a shaped profile.For a corrugated profile across the width of the sheet, the hightemperature gas bearing corrugation wavelength could decrease in unisonwith the sintering shrinkage. In various embodiments, instead of, or inaddition to, shaping during sintering, shaping or forming stations maybe located between sintering zone 22 and cooling zone 24.

In some embodiments, the gap size between bearing surfaces 32 and 34 andthe major surfaces of tape 14 may also be sized to facilitate effectiveheat transfer (e.g., via high levels of conductive heat transfer) frombearings 26 and 28 into tape 14. In such embodiments, because of thehigh levels of heat transfer provided by bearings 26 and 28, system 10allows for heating and sintering using relatively low amounts of energyas compared to furnace or kiln based sintering systems.

In various embodiments, the height of channel 30 (e.g., the distancebetween opposing bearing surfaces 32 and 34) is selected to support tape14 and/or provide the various support functions discussed herein. Invarious embodiments, the height of channel 30 is selected such that thegap size, G1, between bearing surface 32 or 34 and the opposing majorsurface of tape 14 allows for the desired bearing functionality and isbased on the thickness of tape 14. Accordingly, as shown in FIG. 4, theheight of channel 30 is equal to G1 times 2 plus the thickness of tape14. In various embodiments, G1 is between 0.1 μm and 10 mm.

In one embodiment, the height of channel 30 and consequently G1decreases along the length of gas bearings 26 and 28 in the processingdirection. Applicant believes that by providing a wider gap size atinput end 42 of channel 30, insertion of tape 14 into the sintering zonegas bearing may be easier and the decreasing size of the gap along thelength of the gas bearing increases pressure compressing and flatteningthe tape during sintering. In some embodiments, referring to FIG. 13,bearing surfaces 32 and 34 are shaped such that input end 42 is expanded(e.g., having a greater channel height) and output end 44 is expanded(e.g., having a greater channel height) relative to a central portion ofchannel 30. In various embodiments, this shape increases the stabilityof tape 14 and/or sintered material 16 entering or exiting the gasbearing and/or helps isolate the air bearing channel from adjacentprocessing zones.

Referring to FIG. 6, the temperature profile across sintering station 22is shown according to an exemplary embodiment. As shown in FIG. 6,temperature within sintering station 22 generally increases and reachesa maximum toward the center of sintering station 22. As will beunderstood from a comparison of FIG. 6 and FIG. 2, the scale of FIG. 6is substantially less than that of FIG. 2 to highlight the temperaturedistribution across sintering station 22, even though the temperaturevariation across sintering station 22 is relatively small compared tothe temperature various across system 10 as shown in FIG. 2.

Referring to FIG. 7 and FIG. 8, sintered material 16 formed via theprocess and system described above regarding FIGS. 1-6 are shown anddescribed. In general, sintered material 16 is a relatively thin sheetof sintered ceramic material. As noted above, because of the advantagesof the non-contact, gas bearing support sintering process describedabove, sintered material 16 formed by the system and process discussedherein, has high quality opposing major surfaces, in the as-formed state(e.g., without polishing). Further, because of the nature of the supportprovided by the system and process discussed herein, sintered materialswith one or more of these high surface qualities can be produced atlarge sizes previously believed unobtainable using the conventionalsetter board based sintering processes. As will be explained in moredetail below, sintered material 16 formed by the system and process ofthe present disclosure have first and second major surfaces, and eachmajor surface includes at least one surface quality metric that issubstantially the same as the surface quality metric of the other majorsurface, and in specific embodiments, each major surface includesmultiple surface quality metrics that are substantially the same as thesurface quality metrics of the other major surface Applicant believesthat this high level of surface quality consistency is enabled by thenon-contact system discussed herein, at least in part, because system 10allows for both of the opposing surfaces of tape 14 to experienceessentially the same non-contact conditions (e.g., gas content,pressure, temperature, etc.) during sintering. It should be noted thatat least some surface quality features between the opposing majorsurfaces of sintered material 16 may be different from each otherresulting from the process forming tape 14.

Referring to FIG. 7 and FIG. 8, a sheet of sintered material 16 (e.g.,sheet, foil, tape, etc.) includes a first major surface, shown as uppersurface 50, and a second major surface, shown as lower surface 52, whichis opposite of upper surface 50. The sintered article further includes abody 54 of material extending between the first surface 50 and secondsurface 52.

Sheet of sintered material 16 includes a thickness T1 which is definedas a distance between first surface 50 and second surface 52. Sheet ofsintered material 16 includes a width, W1, and a length, L1. Width W1,may be defined as a first dimension of one of first surface 50 and/orsecond surface 52 that is orthogonal to the thickness T1. Length L1, maybe defined as a second dimension of one of first surface 50 and/orsecond surface 52 that is orthogonal to both thickness T1 and width W1.According to an exemplary embodiment, a sheet of sintered material 16 isan elongate thin tape of sintered material. Due at least in part togeometry, some such embodiments are flexible, allowing sheet of sinteredmaterial 16 to bend around a mandrel or spool, such as uptake reel 40,having a diameter of 1 meter or less, 0.7 meters or less, etc., whichmay be beneficial for manufacturing, storage, etc. In other embodiments,sheet of sintered material 16 may be otherwise shaped, such as round,annular, sleeve- or tube-shaped, not have a constant thickness, etc.

According to an exemplary embodiment, length L1 is greater than twicewidth W1, such as at least 5 times, at least 10 times, at least 100times greater. In some embodiments, width W1 is greater than twice thethickness T1, such as at least 5 times, at least 10 times, at least 100times greater. In some embodiments, width W1 is at least 5 millimeters,such as at least 10 mm, such as at least 50 mm. In some embodiments,thickness T1 is no more than 1 cm, such as no more than 2 centimeters,such as no more than 5 millimeters, such as no more than 2 millimeters,such as no more than 1 millimeter, such as no more than 500 micrometers,such as no more than 200 micrometers. In particular embodiments, T1 isas low as 2 micrometers and W1 is greater than 30 mm. According to anexemplary embodiment, as tape 14 is passed through system 10 and allowedto sinter, the sintering occurs nearly uniformly; and length, width andthickness of the sheet may diminish up to approximately 30%. As such,dimensions of tape 14 disclosed herein may be up to 30% greater thanthose described for sintered material 16 discussed herein. Thin tapesmay allow the manufacturing line to operate rapidly because heat fromthe various stages of system 10 can quickly penetrate and sinter suchtapes. Further, thin tapes may be flexible, facilitating bends andchanges in direction along system 10.

According to other exemplary embodiments, thickness T1 is no more than500 micrometers, such as no more than 250 micrometers, such as no morethan 100 micrometers, and/or at least 20 nanometers. According to anexemplary embodiment, sheet of sintered material 16 is a large areasheet such that first major surface 50 and/or second major surface 52have surface areas of at least 10 square centimeters, such as at least30 square centimeters, such as at least 100 square centimeters, and evenexceeding 1000, 5000, or even 10,000 square centimeters in someembodiments. In some embodiments, width W1 that is less than ¼, ⅕, ⅙,1/7, ⅛, 1/9, 1/10 and/or 1/20 length L1. Such geometries may beparticularly useful for certain applications, such as for use of sheetof sintered material 16 as a substrate of a rectilinear battery and/orfor use of sheet of sintered material 16 as a surface for growing carbonnanotubes in an oven, where sheet of sintered material 16 fills surfacesof the oven, yet does not fill substantial volume of the oven.

In various embodiments, sheet of sintered material 16 is thinner than 1mm, wider than 20 cm and longer than 70 cm. In various embodiments,sheet of sintered material 16 is thinner than 1 mm, wider than 30 cm andlonger than 70 cm. In various embodiments, sheet of sintered material 16is a 1 mm thick article wider than 50 cm and longer than 70 cm.

In various embodiments, sheet of sintered material 16 has a width W1greater than 20 cm and a length L1 greater than 70 cm having thicknessesof 750 microns or less and more specifically of 500 microns or less. Invarious embodiments, sheet of sintered material 16 is thinner than 250microns, wider than 20 cm and longer than 70 cm. In various embodiments,sheet of sintered material 16 is thinner than 250 microns, wider than 30cm and longer than 70 cm. In various embodiments, sheet of sinteredmaterial 16 is thinner than 250 microns wider than 50 cm and longer than70 cm.

In various embodiments, sheet of sintered material 16 is thinner than100 microns, wider than 20 cm and longer than 70 cm. In variousembodiments, sheet of sintered material 16 is thinner than 100 microns,wider than 30 cm and longer than 70 cm. In various embodiments, sheet ofsintered material 16 is thinner than 100 microns wider than 50 cm andlonger than 70 cm are achievable.

According to an exemplary embodiment, sheet of sintered material 16 issubstantially unpolished such that first surface 50 and/or secondsurface 52 have a granular profile as shown conceptually in FIG. 8. Thegranular profile includes grains 56 protruding generally outward fromthe body 54 with a height H1 (e.g., average height) relative to recessedportions of the surface at boundaries 58 between grains 56. In specificembodiments, height H1 is at least 25 nanometers and/or no more than 150micrometers, specifically is at least 50 nanometers and/or no more than80 micrometers.

The granular profile is an indicator of the process of manufacturingsheet of sintered material 16 in that the sintered material 16 wassintered as a thin tape in a noncontact environment, as opposed to beingcut from a boule, and that surfaces 50 and 52 where not polished.Additionally, compared to polished surfaces, the granular profile mayprovide benefits to sintered material 16 in some applications, such asscattering light for a backlight unit of a display, increasing surfacearea for greater adhesion of a coating or for culture growth. Applicantbelieves that sheets of sintered ceramic or other materials cut fromboules may not have readily identifiable grain boundaries present onsurfaces thereon, in contrast to sintered material 16 formed asdiscussed herein. Applicant further believes that boule-cut articles maytypically be polished to correct rough surfaces from the cutting.However, Applicant believes that surface polishing may be particularlydifficult or cumbersome for very thin articles of sintered ceramic orother materials, with the degree of difficulty increasing as thethickness of the sintered material decreases and as the surface areas ofsintered material surfaces increases. However, sintered articlesmanufactured according to the presently disclosed technology may be lessconstrained by such limitations because articles manufactured accordingto the present technology may be continuously manufactured in longlengths of tape with high quality surfaces that do not need polishing.Further, dimensions of system 10, as disclosed herein, may be scaled toaccommodate and sinter wider articles, such as having a width of atleast 2 centimeters, at least 5 centimeters, at least 10 centimeters, atleast 50 centimeters.

In contemplated embodiments, the substantially equal surface qualitymetric of first and second major surfaces 50 and 52 is a highlyconsistent level of unpolished surface roughness of both major surfaces50 and 52. In specific embodiments, the unpolished major surfaces 50 and52 both have a roughness (Ra) from 10 nm to 1000 nm across a distance of10 mm in one dimension along the length of the article, such as from 15nm to 800 nm. In specific embodiments, the unpolished major surfaces 50and 52 both have a roughness from 1 nm to 10 μm over a distance of 1 cmalong a single axis.

In contemplated embodiments, the substantially equal surface qualitymetric of first and second major surfaces 50 and 52 is a highlyconsistent defect quantity measurement of both major surfaces 50 and 52.In some such embodiments, the defect quantity measurement is ameasurement of the area of each surface 50 and 52 occupied by defects.In an exemplary embodiment, the surface consistency of both surfaces 50and 52 is such that a total average area of surface defects per squarecentimeter of first major surface 50 is within plus or minus 50% of atotal average area of surface defects per square centimeter of secondmajor surface 52. In such embodiments, “surface defects” are abrasionsand/or adhesions having a dimension along the respective surface of atleast 15, 10, and/or 5 micrometers. In specific embodiments, the area ofa surface defect is the area of a depression or a projection measured ina plane perpendicular to the thickness of sintered sheet 16. In specificembodiments, the defects are at least one of a depression or projectionhaving a depth or height relative to the average position of surfaces 50and 52 that is at least two times the average grain height H1, and inyet other embodiments, the defects are at least one of a depression orprojection having a depth or height relative to the average position ofsurfaces 50 and 52 that is at least 300 μm.

In some other embodiments, the defect quantity measurement is ameasurement of the number of defects present per unit area on eachsurface 50 and 52. In some embodiments, the surface quality is suchthat, on average per square centimeter, major surfaces 50 and 52 bothhave fewer than 15, 10, and/or 5 surface defects that have at least onedimension greater than 15, 10, and/or 5 micrometers. In specificembodiments, the surface quality is such that, major surfaces 50 and 52both have fewer than 3 such surface defects on average per squarecentimeter, and more specifically fewer than one such surface defect onaverage per square centimeter. Accordingly, sintered articlesmanufactured according to inventive technologies disclosed herein mayhave relatively high and consistent surface quality. Applicant believesthat the high and consistent surface quality of sheet of sinteredmaterial 16 facilitates increased strength by reducing sites for stressconcentrations and/or crack initiations. Applicant believes that thislow defect rate on both major surfaces 50 and 52 of sintered material 16is enabled by the noncontact system and process discussed herein.Further, Applicant believes that material sintered while supported by asetter board will typically have at least one surface having a muchhigher defect rate than the other surface resulting from adhesionsand/or abrasions caused as the sintering material contracts whileengaged with the setter board.

In contemplated embodiments, the substantially equal surface qualitymetric of first and second major surfaces 50 and 52 is a highlyconsistent flatness measurement of both major surfaces 50 and 52. Inspecific embodiments, the high level of flatness is achieved in theas-formed article from system 10 without the need for reshaping,grinding, pressing, etc. In specific embodiments, surfaces 50 and 52both have a flatness from about 0.1 μm to about 50 μm over a distance of1 cm along a single axis, such as along the length of sheet of sinteredmaterial 16. Such flatness, in combination with the surface quality,surface consistency, large area, thin thickness, and/or materialproperties of materials disclosed herein, may allow sheets, substrates,sintered tapes, articles, etc. to be particularly useful for variousapplications, such as tough cover sheets for displays, high-temperaturesubstrates, flexible separators, and other applications.

In contemplated embodiments, the substantially equal surface qualitymetric of first and second major surfaces 50 and 52 is a highlyconsistent chemical composition of regions of sintered material 16adjacent to both major surfaces 50 and 52. Applicant believes thatsetter board based sintering process are unable to achieve the very lowlevels of material contaminants on both sides of the sintered articledue to diffusion, adherence, etc. of material from the setter board intothe sintered article. In specific embodiments, first and second majorsurfaces 50 and 52 have a high purity and highly consistent compositionsuch that the chemical composition of material 16 within a depth of 1 μmfrom first surface 50 is at least 99.9% by weight of material of presentin tape 14 (e.g., a polycrystalline ceramic material, syntheticmaterial, binder material) and the chemical composition within a depthof 1 μm from second surface 52 is also at least 99.9% by weight ofmaterial of present in tape 14. In specific embodiments, first andsecond major surfaces 50 and 52 have a high purity and highly consistentcomposition such that the chemical composition of material 16 within adepth of 1 μm from first surface 50 is at least 99.9% by weight of theinorganic material of tape 14 (e.g., a polycrystalline ceramic material,synthetic material) and the chemical composition within a depth of 1 μmfrom second surface 52 is also at least 99.9% by weight of material ofthe inorganic material of tape 14.

In such embodiments, the chemical composition of the regions adjacentthe first and second surfaces are substantial the same as the chemicalcomposition at the midpoint of the thickness of sintered material 16. Insome such embodiments, the chemical composition of material 16 within adepth of 1 μm from first surface 50 is at least 99.9% by weight of theinorganic material of tape 14 (e.g., a polycrystalline ceramic material,synthetic material), the chemical composition within a depth of 1 μmfrom second surface 52 is also at least 99.9% by weight of material ofthe inorganic material of tape 14, and the chemical composition at themidpoint of the thickness is also at least 99.9% by weight of materialof the inorganic material of tape 14.

In yet other embodiments, the high level of chemical compositionconsistency between surfaces 50 and 52 results from the lack of chemicalcontamination typically found in setter board based processes. In suchembodiments, the first surface quality metric and second surface qualitymetric of surfaces 50 and 52 are each a chemical composition of thesintered article within a depth of 0.5 μm from the first and secondmajor surfaces, respectively, wherein the first surface quality metricis substantially the same as the second surface quality metric such thechemical composition within a depth of 0.5 μm from the first surface isat least 99.9% the same as the chemical composition within a depth of0.5 μm from the second surface.

According to exemplary embodiments, sheet of sintered material 16includes polycrystalline ceramic. According to an exemplary embodiment,sheet of sintered material 16 includes (e.g., is, consists essentiallyof, consists at least 50% by weight of) zirconia, alumina, spinel (e.g.,MgAl₂O₄, ZnAl₂O₄, FeAl₂O₄, MnAl₂O₄, CuFe₂O₄, MgFe₂O₄, FeCr₂O₄), garnet,cordierite, mullite, sialon, perovskite, pyrochlore, silicon carbide,silicon nitride, boron carbide, transitional metal borides and carbides,ZrB₂, HfB₂, TiB₂, ZrC, TiC, silicon alumina nitride, and/or aluminumoxynitride. In some embodiments, sheet of sintered material 16 includesion conductors for oxygen ions, Li ions, Na ions, proton conductors,ceramics with low dielectric constants, ceramics with high dielectricconstants, sintered glass ceramics such as cordierite, ceramics andsintered glass and glass ceramics that are porous and ceramics andsintered glass and glass ceramics with no open porosity, translucentceramics and transparent ceramics. In some embodiments, sheet ofsintered material 16 is a metal. In other embodiments, sheet of sinteredmaterial 16 is glass sintered from powder grains. In some embodiments,sheet of sintered material 16 is an IX glass and/or glass-ceramic.Materials disclosed herein may be synthetic.

For example, in some embodiments, sheet of sintered material 16 is madefrom alumina powder having a median particle size diameter of from 50 to1000 nanometers and a BET surface area of from 2 to 30 m²/g. In specificembodiments, sheet of sintered material 16 is made from a tape-castedalumina powder of from 99.5 to 99.995 weight percent alumina and fromabout 100 to about 1000 parts per million of a sintering additive, suchas magnesium oxide. In some embodiments, sheet of sintered material 16is translucent. Sheet of sintered material 16 may have a totaltransmittance of at least 30% at wavelengths from about 300 nm to about800 nm when sheet of sintered material 16 has a thickness of 500 μm orless. In some embodiments, the total transmission through sheet ofsintered material 16 is from about 50% to about 85% at wavelengths fromabout 300 nm to about 800 nm when sheet of sintered material 16 has athickness of 500 μm or less. In some embodiments, diffuse transmissionthrough sheet of sintered material 16 is from about 10% to about 60% atwavelengths from about 300 nm to about 800 nm when the sheet has athickness of 500 μm or less. In contemplated embodiments, sheet ofsintered material 16 may have the above-disclosed transmittancepercentages with a wavelength in the above-disclosed ranges but withother thicknesses, such as other thicknesses disclosed herein. Materialsdisclosed herein other than alumina may also result in such atranslucent sintered article.

In various embodiments, the thin, large and/or high surface qualitybearing ceramic articles enabled by the system and method of the presentapplication have a wide variety of applications. In various embodiments,sintered material 16 is believed to have application as substrates suchas in batteries, on printed circuit boards, as cover sheets fordisplays, such for handheld devices, or the articles may be otherwiseuseful. Sintered material 16 may be used in high power, thin disklasers. Sintered ceramics can be used as hard scratch-resistant coatingsand as armor.

Referring to FIG. 9 and FIG. 10, gas delivery provided by gas bearings26 and 28 are shown and described in more detail, according to exemplaryembodiments. FIG. 9 shows gas bearing 28 with the understanding that gasbearing 26 is formed in substantially the same manner as gas bearing 28.Gas bearing 28 includes a plurality of channels 60 extending widthwiseacross the bearing. Channels 60 deliver pressurized gas from a plenum 64to nozzles 36. Surface 62 allows for attachment of gas bearing 28 to theplenum 64.

As shown in FIG. 10, plenum 64 receives a flow pressurized gas 66 andincludes a port 68 for connecting to a source of pressurized gas. Rearface 70 of plenum 64 attaches to surface 62 of gas bearing 28. Plenum 64includes a flow channel 72 that directs gas 66 through plenum 64 to gasflows 74 that are directed into channels 60 of gas bearing 28. Then thegas flows into channels 60 and out of nozzles 36 where it supports tape14 and sintered material 16 as discussed above. Plenum 64 can be portedto monitor and/or control air pressure supplied to the gas bearing.

Gas bearing 28 includes a plurality of vent channels 76 fed by aplurality of vent openings 78 located in bearing surface 34. Gas 80escaping nozzles 36 is received by vent openings 78. The gas 80 flowsinto vent openings 78 and through vent channels 76 such that exiting gas82 is received by a plenum (e.g., to vacuum draw gas 82) attached torear surface 84 of gas bearing 28.

As shown in FIG. 9, pressurized gases 80 leave by flowing laterally inthe gap between the tape and the gas bearing elements. In this design,the gas accumulates moving from the center of the gas bearing element tothe edges. The rate of flow of gas in the lateral direction is highestnear the edges. The accumulation of gases becomes larger as the width ofthe gas bearing system increases to handle wider ceramic articles. Itcan be advantageous to manage the flow of the departing gases throughthe design of the gas bearing elements. The viscous forces on theceramic due to the flow of the pressurized gas may be minimized or aviscous force can be applied to move the ceramic. The exiting gas 82 canbe directed out of the system. By controlling the venting gas in thismanner, the field of pressure on tape 14 during sintering can be mademore homogeneous and the amount of pressurized gas supplied by the gasbearings to support tape 14 can be reduced or minimized.

Gas bearings 26 and 28 may be structured or configured to facilitatefunctionality described herein. In various embodiments, the spatialpattern of nozzles 36 and/or vent openings 78 may be arranged to improveor optimize venting gas. In various embodiments, the diameter of theholes 36 is greater than 100 μm, specifically greater than 300 μm and insome embodiments greater than 500 μm. In various embodiments, the pitchof holes 36 is less than 10 mm, specifically less than 7 mm and morespecifically less than 3 mm. In various embodiments, holes 36 of lowerair bearing 28 align with holes 36 of the upper air bearing 26. In otherembodiments, the air diffuser holes 36 of air bearings 26 and 28 areoffset from each other.

In various embodiments, the diameter and/length of nozzles 36 and/orvent openings 78 may vary along surface 34 to improve or optimizeventing gas. In various embodiments, surface 34 may include one or moretrench extending across the bearing surface to improve or optimizeventing gas. In some embodiments, gas bearings 26 and 28 may beasymmetrical in design to create a net force that cause movement of tape14 and sintered material 16 such as to convey tape 14 and sinteredmaterial 16 through system 10.

In various embodiments, system 10 and specifically gas bearings 26 and28 are configured for heat recycling. In one embodiment, system 10 andspecifically gas bearings 26 and 28 are configured to reuse at least 30%gas from the gas bearing. In another embodiment, system 10 is configuredto have the gas bearing gas go through a heat exchanger.

Materials for gas bearings 26 and 28 and plenum 64 are selected basedupon temperature and atmospheric requirements of tape 14 and sinteredmaterial 16 and based upon the function of the associated station orzone of system 10. In one embodiment, gas bearings 26 and 28 and plenum64 are made from aluminum. In another embodiment, gas bearings 26 and 28and plenum 64 are made from a bronze 700XX alloy which can operate attemperatures of up to or about 850° C. In another embodiment, gasbearings 26 and 28 and plenum 64 are made from high temperaturenickel-chromium alloys, such as Rolled Alloy 602 CA, 333 or similarwhich can operate at temperatures up to about 1200° C. In anotherembodiment, gas bearings 26 and 28 and plenum 64 are made from preciousmetals like platinum and its alloys with rhodium (e.g., alloys availablefrom Heraeus) which can operate at temperatures of about 1700° C. andare inert in oxidizing atmospheres. In another embodiment, gas bearings26 and 28 and plenum 64 are made from ceramics such as alumina, siliconcarbide, boron carbide, and graphite.

In a particular embodiment, gas bearings 26 and 28 and plenum 64 forbinder removal zone 18 are made from a material such as Bronze 700XX. Inembodiments in which sintering zone 22 is configured to sinter aluminaor zirconia or other ceramic materials that require sinteringtemperatures above 1250° C., gas bearings 26 and 28 and plenum 64 withinsintering zone 22 are made from a high temperature nickel-chrome alloy.In embodiments where higher sintering temperatures are needed, gasbearings 26 and 28 and plenum 64 for sintering zone 22 are made from amaterial that can withstand an oxidizing environment and elevatedtemperatures like a precious metal alloy or ceramic such as siliconcarbide.

AIR BEARING EXAMPLE 1

Referring to FIG. 11 and FIG. 12, an example of using a high temperatureair bearing as part of a process for continuous sintering of aluminatape is shown. In this example, the green tape 14 is 50.8 mm wide and 40μm thick prior to binder removal. It is being fed at a rate of 75 mm/minin the downward direction into opposed stainless steel, high temperatureair bearings for pyrolysis of the organic binder material, such asdescribed above regarding binder removal zone 18. The gap betweenopposing air bearing surfaces is 4.38 mm and the temperature is 500° C.The pressure delivering air in both elements is 10 psi. The advantage ofthe use of an air bearing is visible in the figure. The pressure fromthe opposed air bearings holds the tape 14 flat and limits/prevents warpor distortion as the organic binder is pyrolyzed. After pyrolysis, thetape travels further downward into a furnace 90 at 1050° C. for partialsintering. In this example, the tape is not supported with air bearingsin the furnace 90 during the sintering step nor is it supported by asetter. Instead, the tape 14 supports its own weight so the processremains contactless and free of friction. The partially sintered tapeexiting furnace 90 is flat, width shrinks by 2.6%, and the weight dropsby 8.6%.

AIR BEARING EXAMPLE 2

Referring to FIG. 14 and FIG. 15, an example of a high temperature airbearing, such as air bearings 26 and 28, designed and tested to producesintered articles is shown according to an exemplary embodiment. In thistest, the air bearing shown in FIGS. 14 and 15 was used for a batchsintering test (rather than a continuous process). Referring to FIG. 14and FIG. 15, in this embodiment, the gas diffuser holes, shown as holes36, are arranged in a regularly spaced grid. Holes 36 deliver thesupporting or working gas to the gap of the bearing such that thearticle 14 is supported by the bearing. Holes 36 are formed in a gasdiffuser plate 50, and gas diffuser plate 50 is coupled to the plenumvia a weld joint 52. Applicant has determined that the size and spacingof holes 36 are selected to provide different levels of article supportwithin the gas bearing. In the specific example shown in FIGS. 14 and15, gas diffuser holes 36 have a diameter of 0.5 mm and pitch of 2.54mm.

Preheated gases such as nitrogen or air are supplied through tubing 54made from the same or similar metal that the plenum of air bearing 26 ismade from, and the supplied gas then flows out the diffuser holes 36into the gap between the opposing air bearings 26 and 28. In thespecific example shown in FIG. 15, nitrogen was the gas supplied to airbearings 26 and 28 to support the zirconia tape material 14 within theair bearing gap.

Referring to FIG. 14, each of the air bearings 26 and 28 include agroove or trench 56 machined into air diffuser plate 50 adjacent itsouter perimeter. In the particular embodiment shown, groove 56 is 4 mmwide and 1 mm deep. Groove 56 provides a stable area to engage withspacers or stand-offs 35 that support air bearings 26 and 28 relative toeach other and that defines and maintains the gap spacing betweenopposing air bearing surfaces. In addition to maintaining the gapspacing, stand-offs 35 also provide a lateral constraint to the articlebeing sintered which acts to block the material being sintered fromsliding laterally out of the gap between the opposing air bearingsurfaces. In various embodiments, stand-offs 35 may be formed from aceramic or metal, and in a specific embodiment is formed from an aluminamaterial.

As shown in FIG. 14, at least one of the air bearings 26 or 28 includesa mounting portion 58 that includes a recess or other couplingarrangement for coupling of tubing 54 to the air bearing. In addition,at least one of the air bearings 26 or 28 includes a mounting portion 60that includes a recess or other coupling arrangement for receivingtubing of a pressure sensing device. In the specific embodiment shown,air bearings 26 and 28 have a width W3, which is between 100 mm and 150mm and specifically is 127 mm.

FIG. 15 is a side view of a pair of air bearings 26 and 28, eachgenerally having the air hole pattern as shown in FIG. 14. As shown inFIG. 15, in this particular example, the opposing air bearings 26 and 28are separated by alumina stand-offs 35, and the material to be sintered,tape 14, is green zirconia (3-YSE) tape. The tape 14 is suspendedbetween bearings 26 and 28 by the centering forces from the pressure ofthe gas flowing into the air bearing gap through holes 36. In thisexample, the air bearing 26 and 28 were constructed from RA-333nickel-chromium alloy sold by Rolled Alloys of Temperance, Mich. It iscapable of operation to approximately 1200° C. In this example todeliver the sintering heat, opposed air bearings 26 and 28 wereinstalled inside an electrically powered furnace.

For high temperature sintering, Applicant has determined that diffuserplates 50 of sufficient thickness perform well. In specific embodiments,diffuser plate 50 of air bearings 26 and/or 28 is at least 2 mm thick,specifically at least 4 mm thick and more preferably at least 6 mmthick.

As discussed above, the spacing of the opposing air bearings 26 and 28defines a gap, shown as G2, in FIG. 15. Based on Applicant's testing,Applicant has determined that the gap sizing relates to varioussintering properties. In particular, as discussed below, Applicant hasdiscovered that gap sizing, particularly small gap sizing, influencesthe position of the article within the bearing during sintering and alsomay influence sintered article properties such as flatness andtransparency. Thus, in the example shown in FIG. 15, G2 is selected suchthat G2 minus the thickness of the tape 14 is less than 5 mm,specifically less than 2 mm and more specifically less than 1 mm.

In various embodiments, the material used for the stand-offs 35 isselected for compatibility with the air bearing materials. In a specificembodiment, stand-offs 35 are formed from alumina for contact with hightemperature nickel-chrome alloys or precious metals like platinum, thatmay be used to form air bearings 26 and 28. In specific embodiments,stand-offs 35 include vents for exit of the gas injected into the airbearing gap through the holes 36. The spacing of the vents withinstand-offs 35 may be similar to the pitch of the diffuser hole 36. Insome embodiments, the stand-offs include notches along the lateral edgesof each stand off to provide venting.

In some embodiments, outer, opposing surfaces of diffuser plates 50 ofair bearings 26 and 28 are flat to provide a flat article after presintering or after sintering. While in other embodiments, outer,opposing surfaces of diffuser plates 50 of air bearings 26 and 28 may becurved or have other shapes to impart that shape to the pre-sintered orfired article. In operation, pressure and gas flow through holes 36 intothe air bearing gap is controlled to provide the desired support to tape14 during sintering. In an exemplary embodiment, the pressure in theplenum of each air bearing 26 and 28 is less than 40 psi, specificallyless than 20 psi, and more specifically less than 5 psi. In variousembodiments, the rate of gas flow to the bearing per unit area of activediffuser plate surface is less than 0.1 SCFM/cm² and more than 0.002SCFM/cm². In other embodiments, the rate of gas flow to the bearing perunit area of active diffuser plate surface is less than 0.04 SCFM/cm²and more than 0.004 SCFM/cm².

SINTERING TEST EXAMPLE 1

Utilizing the air bearing arrangement shown in FIG. 14 and FIG. 15,zirconia tape was sintered utilizing different bearing gap spacing, G2,and based on this testing, Applicant determined that gap spacinginfluences various properties of the sintered article. Referring toFIGS. 16 and 17, zirconia (3-YSE) sintered tape having a thickness of 20μm and post sintering width of 80 μm, was sintered via air bearings 26and 28 using different gap sizes, G2. Both materials were heated withinair bearings 26 and 28 for four hours at 1150° C. In this test, the gasflow to the air bearings was nitrogen at 1 SCFM.

The sintered zirconia tape shown in FIG. 16 was sintered in an airbearing having a 0.5 mm gap, G2, and was suspended throughout thesintering process (i.e., there was no contact with the stand-offs ormetal of the air bearing. As shown in FIG. 16, utilizing this gapspacing the sintered zirconia tape was mostly flat and is sintered totranslucent indicating a low level of porosity.

The sintered zirconia tape shown in FIG. 17 was sintered in an airbearing having a 1.5 mm gap, G2. As compared to the tape shown in FIG.16, the tape sintered in the 1.5 mm gap air bearing is visibly morewrinkled. In addition, the tape shown in FIG. 17 moved parallel to thefaces of the air bearing diffuser plates and contacted two of thestand-offs. Applicant believes this movement to have occurred becausewhen tape is sintered in the larger, 1.5 mm gap, the tape has more spaceto move and the centering forces on the tape provided by the gasdelivered to the gap are lower. The forces generated by contact betweenthe tape and the stand-offs and the thin flexible nature of the 3-YSEtape has led to buckling at the contact sites. Regions of the buckledzone that contacted the metal surface of the air bearing diffuser plateare dark in color due to chemical interdiffusion.

SINTERING TEST EXAMPLE 2

As shown in FIG. 18 and FIG. 19, similar results were obtained with 70μm thick alumina tape after partial sintering at 1150° C. for 6 hours inthe same air bearing under the same conditions as those discussed aboveregarding FIGS. 16 and 17. FIG. 18 shows alumina tape partially sinteredwithin an air bearing having a 0.5 mm gap, and FIG. 19 shows aluminatape partially sintered within an air bearing having a 1.5 mm gap. Thealumina tape shown in FIG. 18 is flatter after partial sintering in the0.5 mm gap as compared to the alumina tape shown in FIG. 19. Inaddition, the zone of contact with the stand-offs shown in FIG. 18 issmaller than the similar contact zone shown in FIG. 19. More than 90% ofthe tape was pre-sintered without contact to either of the bearingfaces.

Based on this testing, Applicant has hypothesized that contact of thealumina tape to the air bearing face near the stand-offs appears to bedue to inadequate pressure to support the tape by the working gas inthis region. Nitrogen gas flow is diverted away from the stand-offs, andthere is visible oxidation of the metal diffuser plates due to backdiffusion of oxygen from air under and over the stand-offs. Otherregions of the air diffuser plate are covered by nitrogen and thus lessoxidized. Thus, based on this testing, Applicant has determined that, inat least some embodiments, stand-offs with periodically spaced vents orholes with spacing similar to the pitch of the different holes wouldmake flow of nitrogen exiting the air bearing more uniform around theperimeter, and Applicant believes that increased uniformity wouldprovide better support for the tape within the air bearing gap,reduction/elimination of contact with the bearing faces near thestand-offs, and/or improved tape flatness.

SINTERING TEST EXAMPLE 3

Referring to FIG. 20 and FIG. 21, as another example, Pyrex 7761 fritpowders were calendared to a thickness of about 70 μm. The ribbon washeated in the opposed air bearings shown in FIG. 14 and FIG. 15 at 750°C. for 2 hrs. with a flow of 1 SCFM of gas (e.g., air or nitrogen) tothe bearings. The gap spacing, G2, was 0.5 mm. FIG. 20 shows the 70 μmthick Pyrex tape before heating/sintering, and FIG. 21 shows the sametape after heating/sintering. As shown in FIG. 21, flat and transparentsintered glass was obtained. Applicant notes that the holes in the tapeshown in FIG. 21 were formed from handling introduced after sinteringand not from the sintering process.

SINTERING TEST EXAMPLE 4

Referring to FIG. 22 and FIG. 23, Applicant has determined that flatnessof the sintered tape can be affected and improved by application of asmall amount of externally applied stress, in at least some embodiments.Ribbons of 40 μm thick zirconia (3-YSE) tape that are 40 mm wide (green)and approximately 300 mm long were fired at 1175° C. for 4 hrs. with thecenter section of the ribbon suspended between the opposed air bearingsof FIGS. 14 and 15 without contacting the air bearing surfaces. FIG. 22shows a tape sintered with 75 kPa of tension applied during sintering,and FIG. 23 shows the tape sintered without tension. The ribbon sinteredwith applied tension as shown in FIG. 22 is visibly flatter than the onewithout, shown in FIG. 23. In addition, ribbon sintered with appliedtension as shown in FIG. 22 is more translucent than the one without,shown in FIG. 23.

Unless otherwise expressly stated, it is in no way intended that anymethod set forth herein be construed as requiring that its steps beperformed in a specific order. Accordingly, where a method claim doesnot actually recite an order to be followed by its steps or it is nototherwise specifically stated in the claims or descriptions that thesteps are to be limited to a specific order, it is in no way intendedthat any particular order be inferred. In addition, as used herein, thearticle “a” is intended to include one or more than one component orelement, and is not intended to be construed as meaning only one.

It will be apparent to those skilled in the art that variousmodifications and variations can be made without departing from thespirit or scope of the disclosed embodiments. Since modifications,combinations, sub-combinations and variations of the disclosedembodiments incorporating the spirit and substance of the embodimentsmay occur to persons skilled in the art, the disclosed embodimentsshould be construed to include everything within the scope of theappended claims and their equivalents.

What is claimed is:
 1. A process of forming a sintered article,comprising: heating a green portion of a tape of inorganic material inorganic binder, at a removal zone, to a temperature sufficient topyrolyze the organic binder, and thereby removing organic binder fromthe portion of tape; horizontally conveying the portion of tape withorganic binder removed from the removal zone to a sintering zone,wherein the tape of inorganic material extending between the removalzone and the sintering zone is unsintered inorganic material; andsintering inorganic material of the portion of tape at the sinteringzone by heating the inorganic material to a temperature at or above asintering temperature of the inorganic material such that the inorganicmaterial is sintered, wherein the tape simultaneously extends throughthe removal and sintering zones and comprises portions thatcontemporaneously include inorganic material with organic binder,unsintered inorganic material with organic binder removed, and at leastpartially sintered inorganic material.